1. Field of the Invention
This invention relates to a solid-state image pickup device, such as a CMOS image sensor, and, more particularly to, a pattern layout for achieving high integration in the horizontal direction of the solid-state image pickup device.
2. Description of the Related Art
In digital cameras and camera phones, a CMOS image sensor is used as an image pickup device. A low-voltage, low-power-consumption CMOS image sensor suitable for this type of device has been described in, for example. U.S. Pat. No. 6,091,449. In the CMOS image sensor, unit cells are arranged in a matrix. Each of the unit cells is composed of photodiodes corresponding to pixels, a transfer transistor, a reset transistor, an address transistor, and a driving transistor.
The photodiodes are arranged in a square-lattice pattern. The signal charges generated and accumulated at these photodiodes are transferred to the floating junctions and are accumulated there. The transfer of the signal charge is carried out by supplying a transfer pulse to the gate of the transfer transistor (or the transfer gate) to form a channel (or open the transfer gate). At this time, the transfer gates arranged in the same row to which a transfer pulse is supplied are also opened, with the result that the signal charges at the photodiodes connected to these transfer gates are also transferred to the floating junctions.
Before the transfer of the signal charge, the potential at each floating junction has been initialized (or reset) to a reset drain voltage by the reset transistor. The voltage level of the floating junction varies as a result of the signal charge flowing in the floating junction. The floating junction is connected to the gate of a driving transistor (or driving gate). A variation in the potential of the floating junction causes the channel potential to be modulated.
Then, an address signal is supplied to the gate of the address transistor (or the address gate), thereby connecting the driving transistor whose channel potential is modulated to a signal line. Load transistors are connected to these signal lines. A signal corresponding to the signal charge is output from a horizontal read circuit to the outside in a time-series manner.
After the reading of one line has been completed, the floating junctions are reset again to the reset drain voltage. Thereafter, the transfer gates are opened, thereby transferring the signal charges generated and accumulated at the photodiodes to the floating junctions. Then, the above-described operations are repeated, thereby reading one line of signal charges.
The signal charges at the photodiodes in the next pixel line and those at the photodiodes in the pixel line after the next are read by sharing the floating junctions. The same operations as described above are repeated, thereby reading all of the pixels.
In U.S. Pat. No. 6,091,449, two pixels vertically arranged share an output circuit. Specifically, the signal charges at two photodiodes arranged adjacently in the vertical direction are transferred to a common floating junction and are read by the output circuit. Therefore, this configuration is suitable for high integration.
Although the configuration is suitable for high integration in the vertical direction, it is difficult to achieve high integration in the horizontal direction because pixels and output circuits are arranged in the horizontal direction. When the configuration is applied to a digital camera, the pixel pitch in the horizontal direction must be made equal to the pixel pitch in the vertical direction. Therefore, even if only either the horizontal pitch or the vertical pitch is reduced, high integration cannot be realized. Moreover, information on the spacing between pixels in the horizontal direction corresponding to the output circuit cannot be acquired.
Furthermore, in the conventional image sensor, since the pixels and output circuits are arranged in the horizontal direction, the opening for the pixels is vertically long. Even in a CCD area sensor, since there are CCD registers in the horizontal direction of the pixels, the opening for the pixels is also vertically long.
In general, the horizontal-to-vertical ratio of the optical system of a camera is 3:4, that is, horizontally long. Therefore, when the pixels in the peripheral part of the light-sensitive surface are considered, light coming in through the lens and entering the light-sensitive surface of the sensor enters the end of the pixels in the horizontal direction more diagonally than the end of the pixels in the vertical direction. Accordingly, the output of the peripheral pixels is less than that of the pixels in the central part, resulting in so-called shading. As described above, when the opening for the pixels is vertically long, the light-sensitive surface is more liable to be affected by shading.